Ceramic compositions of crystallizing glasses are known which, when mixed with non-crystallizing glasses, form green tape compositions which can be adhered to metal core support substrates such as KOVAR.RTM. a trademark of Carpenter Technolory.
KOVAR.RTM. is a Fe/Co/Ni alloy. Such alloys include an alloy containing 53.8 weight percent of iron, 29 weight percent of nickel, 17 weight percent of cobalt and 0.2 weight percent of manganese. These alloys display a sharp change in their coefficient of expansion at certain temperatures. Metal KOVAR.RTM. plates are available that are plated with a 0.025 mm (1 mil) coating of copper and a 0.025 mm (1 mil) coating of nickel on each side of the KOVAR.RTM. core, to a total thickness of 0.025 inch. These plates have a thermal coefficient of expansion (TCE) of 5.8 ppm/.degree.C. (RT to 300.degree. C.) and a thermal conductivity (z or thickness direction) of 21.8 Watt/m.degree.K. This TCE is closely matched to gallium arsenide in the RT to 300.degree. C. range.
As employed for multilayer support substrates, the copper and nickel plated substrates are heat treated in air to oxidize the nickel coating, and then glazed with a CaO-Al.sub.2 O.sub.3 -ZnO-B.sub.2 O.sub.3 bonding glass. Such bonding glasses are known and are made by mixing a suitable glass powder with an organic binder and a solvent to form a screen printable ink which is applied to the support. Preferably the bonding glass is applied in two applications to a thickness of 40-70 microns. The glass is dried and densified by firing at 700.degree.-800.degree. C. To improve the adhesion of the bonding glass to KOVAR.RTM. supports, about 6% by weight of copper powder can be added to the bonding glass. These prepared KOVAR.RTM. support plates are used herein and when co-laminated to low firing temperature green tape compositions, they prevent shrinkage of the ceramic layers in the x and y dimensions.
To form the green tape compositions, crystallizing glasses made from glass systems of the ZnO-MgO-B.sub.2 O.sub.3 -SiO.sub.2 type, are mixed together with non-crystallizing glasses and minor amounts of oxide fillers. For example, crystallizing glasses containing 20-55% by weight of ZnO; 20-28% by weight of MgO; 10-35% by weight of B.sub.2 O.sub.3 ; and 10-40% by weight of SiO.sub.2, have a thermal coefficient of expansion (TCE) matched to kovar and low dielectric loss properties that are compatible with microwave components; however, they have a low crystallization temperature which inhibits densification of the glass on firing.
These glasses advantageously can be mixed with a lead-based, non-crystallizing glass. Suitably these non-crystallizing glasses contain from 30-80% by weight of PbO, 15-50% by weight of SiO.sub.2, up to 10% by weight of Al.sub.2 O.sub.3, up to 15% by weight of B.sub.2 O.sub.3 and up to 10% by weight of ZnO.
However, when the crystallizing glasses are mixed with lead-based, non-crystallizing glasses, the TCE is lowered and the dielectric loss properties are increased. The lateral shrinkage (x and y) of the mixed glasses is still high during firing of the glass mixtures as well. The addition of minor amounts of oxide fillers, such as quartz, alumina, forsterite and the like, reduces the lateral shrinkage during firing. Thus these modified ceramics have the desired dielectric properties, low shrinkage during firing, and a TCE matched to KOVAR.RTM..
Table I below sets forth the desired properties for the fired glass-ceramic compositions useful herein:
TABLE I ______________________________________ TCE (25-300.degree. C.) 5-7 ppm/.degree.C. Dielectric Constant @ 1GHz 5-10 .+-. 3% Dielectric Loss @ 1 GHz (tan .delta.) .ltoreq.0.002 Volume Resistivity .gtoreq.10.sup.13 ohm-cm Surface Resistivity &gt;10.sup.12 ohm-cm Chemical Durability Resistant to acids, alkalis and plating solutions Buried Conductor Resistance &lt;4m.OMEGA./square Via conductor Resistance &lt;1m.OMEGA./via ______________________________________
The glasses are formed into green tapes in known manner, i.e., by slurrying the glass powders with a resin binder and suitable dispersants and solvent. These slurries are cast to form green tapes. Conductive inks can be screen printed onto the green tapes to form circuit patterns. Several green tapes can be aligned and stacked and laminated under pressure. Via holes punched in the green tapes and filled with conductor inks provide a conductive path between the circuit patterns. These laminated green tape stacks are then aligned with a support substrate coated with a bonding glass, and co-laminated, also under pressure. Since shrinkage occurs mainly in the thickness (z) dimension during firing, to remove the organics and densify the glasses, the circuitry is not disturbed during firing and close tolerances can be maintained.
Further, these ceramics are compatible with low melt temperature conductive inks, such as silver-based inks, used to form the electrically connected circuits on the various layers and to form bond pads and the like. Thus the ceramic circuit boards as described have low dielectric loss properties and are useful for use with microwave/digital packaging.
However, when multilayer green tape stacks that are over about 2 mm in thickness after firing are made, the shrinkage can no longer be closely controlled, and in addition the multilayer stack tends to de-laminate from the metal support when fired. Thick multilayer circuits boards are required when RF components, such as filters, are to be embedded in the ceramic layers. Thus it would be highly desirable to be able to form multilayer printed circuit green tape layers on a metal support which, when fired, are over about 2 mm in thickness and that retain the low dielectric loss properties and low shrinkage in the x and y dimensions of the above-described supported multilayer ceramic circuits.